The Kyokushin Oath
"We will train our hearts and bodies, for a firm unshaken spirit.
We will pursue the true meaning of the Martial Way, so that in time our senses may be alert.
With true vigor, we will seek to cultivate a spirit of self-denial.
We will observe the rules of courtesy, respect our superiors, and refrain from violence.
We will follow our spiritual path, and never forget the true virtue of humility.
We will look upwards to wisdom and strength, not seeking other desires.
All our lives, through the discipline of Karate, we will seek to fulfill the true meaning of the Kyokushin Way."
KYOKUSHIN
Kyoku meaning "Ultimate".Shin meaning "Truth" or "Reality".
Kyokushin the "Ultimate Truth" is the name of the style of karate founded by Sosai (founder) Masutatsu Oyama. It was born in the determination for the pursuit of ultimate truth of mind, technique, and body. Kyokushin is known as "the strongest karate" not only because of the incredible frets of strength and endurance but because of the combination of constant hard training, meditation, determination, repeated practice, and participating in tournaments.
The instructor and its students of Kyokushin all must have taken part in sparring. Unlike other forms of Karate, Kyukushin demands the black belts spar without any gloves or safety gear on. Each fighter is expected to be not only strong, but be able to take the hits as they come. One of the words that is heard mostly in a kyokushin dojo is "osu" which means patience, respect and appreciation. Thus the word "Osu" reminds kyoukshin players of these indispensable qualities.
After Mas Oyama passed away, the World Karate Organization has taken on the mission to spread Kyokushin Karate throughout the world. Kyokushin started to spread to more than 120 countries, and registered players exceed 10 million making it one of the largest full contact karate organizations in the world. One of Mas Oyama’s philosophies that is heard mostly in Kyokushin is "Real fighting cannot exist without proof, proof cannot exist without trust, trust cannot exist without respect." Through rigorous training this martial art has been known as both dangerous and difficult to master.
The Spirit Of "OSU"
Osu is the one word that you'll hear the most in a Kyokushin dojo or at a Kyokushin tournament. When you enter or leave the dojo, you bow and say "Osu". When you greet a fellow Kyokushin karateka, you say "Osu" instead of "hello". When you respond to an instruction or question in class, you say "Osu" instead of "yes" or "I understand". When performing kihon waza (basic techniques) in class, each technique is often accompanied with a loud "Osu". When practicing jiyu kumite (free fighting) in class and your opponent lands a good, hard technique, you say "Osu" to acknowledge your opponent's skill. As a measure of respect, knockdown fighters at a tournament bow and say "Osu" to the front, to the referee and to each other, before and after the fight. Osu is used in many situations and seems to mean a lot of things. But what does it really mean?Osu is a contraction of the words:Oshi meaning "Push", andShinobu meaning "to Endure".It means patience, determination and perseverance. Every time we say "Osu", we remind ourselves of this. Kyokushin training is very demanding. You push yourself until you think you've reached your limit. First your body wants to stop, but your mind keeps pushing you. Then your mind wants to stop, but your spirit keeps you going. You endure the pain. You persevere. That is Osu.
There is a saying in Japan, "Ishi no ue ni sannen." Translated, it means "Three years on a rock." This saying symbolizes the need to persevere at all times. It is one of the most important philosophies in Kyokushin karate. Kyokushin is an art offering many things according to the immediate and long term aims of the trainee. Ultimately, one realizes that transcending the kicks, the punches, and the kata, there is a special spirit in the heart of the participants. It teaches them to face the demands of daily life with a mature and enduring attitude. A budo-ka is not easily shaken by the blows of adversity, realizing that for a person to draw near to their true potential, a never- say-die spirit of perseverance is required.This strength of character develops in hard training and is known as osu no seishin (the spirit of Osu).
As Shihan Cameron Quinn wrote in his book:
The word Osu comes from oshi shinobu, which means "to persevere whilst being pushed". It implies a willingness to push oneself to the limits of endurance, to persevere under any kind of pressure.The single word Osu captures most accurately the ultimate in what the art of karate, particularly Kyokushin, has to offer. One who is truly able to manifest the spirit of Osu in every word, thought, and action may be regarded as wise and brave. Training should first and foremost be approached in the spirit of Osu. One's daily life, and the responsibilities it holds, would be more completely lived if addressed in the spirit of Osu.Even for the beginner, who is conscious of his lack of training and does not necessarily want to face the demand of training, it is enough merely being aware that through perseverance and the will to continue, there comes great physical, mental, spiritual, and emotional gains. All that is needed is that special determination.In other words, Osu.

OSU!!
Small note: For a better insight I suggest you watch the movie "Fighter in the wind"